1. Field of the Invention
The present invention relates to a CCD (Charge Coupled Device) image sensor used in a television camera and especially to a solid state image sensor which reduces excess charge overflow.
2. Description of the Related Art
Recently, a CCD image sensor that is compact, light, and has long life has been widely used as an alternative to an image pick-up tube. In particular, an interline transfer CCD, which has photodiodes and vertical transfer CCD's arranged two-dimensionally in an image plate, is used for an individual video recorder or a handy camera for broadcasting and so on.
FIG. 1(a) and FIG. 1(b) show sectional views of a structure of one pixel (picture element) in a conventional solid state image sensor.
As shown in FIG. 1(a), an n-type diffusion region 100 is formed on a p-type Si substrate 102 to form photodiode 130. An n-type vertical CCD channel region 104 and an overflow drain 106 are also formed on Si substrate 102. N-type vertical CCD channel region 104 and overflow drain 106 are formed on opposite sides of n-type diffusion region 100. A transfer electrode 108, which also functions as a read out electrode, is formed over vertical CCD channel region 104 on a gate insulating layer 110. An overflow charge control electrode 112 is formed on gate insulating layer 110 between n-type diffusion region 100 and overflow drain 106. A planarizing second insulating layer 114 is formed on the surface of Si substrate 102 and an optical shield 116 is formed on portions of second insulating layer 114 such that an opening 118 is formed above n-type diffusion layer 100. In the sensor, incident light (h.nu.) passes through opening 118 of optical shield 116 and is photoelectrically-converted to signal charge which is stored in photodiode 130. The signal charges are transferred to vertical CCD channel region 104 through a field shift gate region 120 formed under transfer electrode 108 between channel region 104 and diffusion region 100. The signal charges are then sequentially transferred within channel region 104 to output circuitry (not shown). Excess charges generated by a strong incident light are extracted to overflow drain 106 through overflow extraction region 122 beneath overflow charge control electrode 112.
FIG. 1(b) shows another conventional image sensor including a so-called vertical overflow drain structure. That is, a p-type well diffusion layer 102a, whose thickness is thinner under n-type diffusion region 100 and is thicker under vertical CCD channel region 104, is formed on an n-type Si substrate 102, instead of forming an overflow charge control electrode. Excess charges are extracted through the thin portion of p-type well diffusion layer 102a into n-type substrate 102b.
FIG. 3 shows an input/output characteristic graph of the pixel shown in FIG. 1. Specifically FIG. 3 illustrates output current vs. amount of incident light of one CCD pixel as shown in FIG. 1(a) and FIG. 1(b). Within range 204 along the abscissa of the graph shown in FIG. 3, the amount of incident light is low and the output current of the pixel has a substantially linear relationship with the amount of incident light. But if the incident light exceeds a specific amount 206, i.e., the region 202 of the graph, the graph levels off such that the output is saturated. That is, the output current is substantially constant and independent of the amount of incident light. The output current becomes saturated because some of the charges generated by the incident light go over the potential barrier of overflow extraction region 122 and are extracted by the overflow drain. The first region 200 is determined by a difference in the "heights" of potential barriers of field shift region 120 and a potential of overflow extraction region 122 during the reading out operation.
FIG. 2 shows a plot of voltage potential vs. distance along lines X-Y in FIG. 1(a). Numbered portions of the plot designate the potential in corresponding numbered regions of FIG. 1(a). The plot denoted by the solid line illustrates potentials in regions 104, 120, 100, 122 and 106 when the CCD pixel of FIG. 1(a) is in a charge storage state, i.e., when charges are stored in photodiode 130. In the charge storage state, charges isolated in a potential well in n-type region 100 by the potential barriers of field shift gate region 120 and overflow extraction region 122. That is, the potentials in field shift gate region 120 and overflow extraction region 122 are such that charges in the potential well cannot be transferred from n-type region 100. In the charge storage state, the "height" of the potential barrier of field shift gate region 120 is greater than that of overflow extraction region 122.
In a read out state of the CCD pixel shown in FIG. 1(a), a voltage is applied to transfer electrode 108 to lower the potential barrier of region 120. Accordingly, as shown by a dotted line in FIG. 2, charge stored n-type region 100 "spills over" into n-type CCD channel region 104 and is subsequently read out to output circuitry.
However, the "heights" of the potential barriers of the field shift region 120 and overflow extraction region 122 is given by a function of several variables, such as the impurity concentration within these regions and the voltages applied to transfer electrode 108 and overflow charge control electrode 112. Accordingly, the difference between the "heights" of the potential barriers in these regions can vary from pixel to pixel in the CCD. In a pixel which has a small difference in the "heights" of the potential barriers of regions 120 and 122, linear region 200 may be narrow and the output current associated with the second leveled off or saturated region may be lower. In a pixel which has a large potential difference, the first region 200 may be wide and the output current of the second leveled off or saturated region may be excessive. Thus, a CCD having many pixels in which the difference in the "heights" of the potential barriers is not uniform also has a limited dynamic range because excessive amounts of signal charge can traverse the potential barrier "height" of overflow extraction region 122, for example, and are not read out. These charges therefore do not contribute to formation of the image. Accordingly, the image generated by such a CCD may be defective.
The conventional CCD pixel shown in FIG. 1(b) also suffers from problems similar to those described above in regard to the CCD pixel shown in FIG. 1(a).
Another conventional image sensor, shown in FIG. 10 is described in U.S. Pat. No. 4,912,560. A p-type well 300 is formed on a surface of n-type substrate 302. An n-type region 304 is disposed in p-type well 300 to form storage (photo) diode 305. A CCD channel region 306, and n-type region 308 for injecting and extracting charges from channel region 306 are formed in p-type well 300. N-type region 308 and the surrounding p-well constitute a diode for the injection and extraction of charge as will be discussed below.
A pull out electrode 310 is provided in contact with n-type region 304. A pixel electrode 312 is formed over storage diode 305 and CCD channel region 306. In addition, a photoelectric conversion layer 314 and a transparent electrode 316, to which a voltage is applied, are stacked on pixel electrode 312. Element 318, shown in FIG. 10, is a gate for extracting charges and element 319 is a read out gate.
In this type of image sensor a residual image can form as a result of residual charges. Residual charges are those signal charges which are not read out and remain in CCD channel region 306 or in photoelectric conversion layer 314, even though most of the signal charge has been read out through photoelectric conversion layer 314, n-type region 304 of storage diode 305, and CCD channel region 306. To prevent formation of the residual image, after signal charges are read out and before subsequently generated signal charges are stored in storage diode 305, bias charges are injected from injecting/extracting diode 307, through CCD channel region 306, into storage diode 304. The bias charges are then transferred to photoelectric conversion layer 314. The bias charges along with any residual charges are then extracted from photoelectric conversion layer 314, through storage diode 304 and CCD channel region 306, back to injecting/extracting diode 307. Extraction of these residual charges inhibits formation of the residual image.
Injection and extraction of bias charge, as described above, is performed after signal charges are read out and before storage of subsequently generated signal charges. The injection and extraction of bias charge occurs during the vertical blanking t.sub.1 period (see FIG. 11) which is determined by the TV format. Photoelectric generation and storage of signal charges are carried out during the effective vertical period t.sub.2, also shown in FIG. 11.
FIGS. 12(a) and 12(b) show potentials in the semiconductor regions of FIG. 10. In FIGS. 12(a) and 12(b), the hatched areas indicate the presence of charge carriers (i.e., electrons). Level A in FIGS. 12(a) and 12(b) represents the potential level at the surface of field shift region 317 when a read out potential is applied to gate 319 to read out charge stored in storage diode 305. As seen in FIG. 12(a), when the image sensor is exposed to moderate light intensities, a corresponding amount of charge photoelectrically generated in photoelectric conversion layer 316 is collected or stored in n-type region 304, pull-out electrode 310 and pixel electrode 312. However, when the image sensor is exposed to high intensity light, as seen in FIG. 12(b), only a maximum amount of charge Q.sub.s can be stored in n-type region 304 and electrodes 310 and 312. Q.sub.s may be determined from the following formula: EQU Q.sub.s =(Potential at level A--Potential at level B).times.C.sub.s ;
where the potential at level B is the potential of transparent electrode 316 and C.sub.s is the capacitance of storage diode 305.
Since the potential at level A is a function of several process dependent variables, such as the thickness of the oxide layer overlying the field shift region and the concentration of doping impurities within this region, the potential at level A can vary from pixel to pixel. If the potential at level A in a particular pixel is high, Q.sub.s for that pixel is correspondingly high and the amount of charge transferred to the CCD channel region from the storage diode can be excessive when the image sensor is exposed to high intensity light. Accordingly, the CCD pixel shown in FIG. 10 can have an input/output characteristic similar to that shown in FIG. 3 which suffers from an excessive output saturation current due to the high Q.sub.s. Thus, the pixel shown in FIG. 10 also has a reduced dynamic range.
Thus, in both of the conventional CCD image sensors described above, excess charge which is generated in response to a high input light intensity causes a deviation in the maximum or saturated output current of each pixel of the image sensor. Accordingly, the image generated by the image sensor can be defective.